1. Field of the Invention
The present invention generally relates to nonvolatile semiconductor memory devices, and particularly relates to the stacked-layer structure of a nonvolatile SRAM cell where a ferroelectric capacitor is provided as a backup for stored data. The present invention also relates to a method of making such stacked-layer structure.
2. Description of the Related Art
Conventional latch circuits are comprised of volatile circuits. When power is turned off, stored data is lost. In recent years, schemes have been proposed for nonvolatile SRAMs (static random access memory) (e.g., Non-patent Document 1 and Non-patent Document 2). In nonvolatile SRAMs, a ferroelectric film (PZT: Pb(Zr, Ti)O3, SBT: SrBi2Ta2O9) is attached as a varying capacitance to a latch circuit that is used as a memory cell.
In such nonvolatile SRAMs, stored data is changed into nonvolatile data as a residual polarization of the ferroelectric capacitor prior to power-off. Such data is retrieved as complementary data of the memory-cell latch circuit upon the power-on. A ferroelectric capacitor in the conventional stacked-layer structure of a nonvolatile SRAM is formed above the layers in which metal wiring such as bit lines are provided. Such order of stacked layers achieves area size that is compatible to that of SRAM cells.                [Non-patent Document 1] Proceeding of IEEE2000 Custom Integrated Circuits Conference, pp. 65-68, May, 2000.        [Non-patenting Document 2] TECHNICAL REPORT OF IEICE. SDM 2001-128, ICD2001-51, May, 2000.        
At present, spin coating or sputtering is known as a method that achieves the stable growth of a ferroelectric film such as PZT. 600 degrees Celsius is known to be a preferable temperature for stable growth. Because of this, the process of growing a ferroelectric film is preferably performed before the process of forming metal wiring made of aluminum (AL) or copper (Cu), which have a melting point lower than the growth temperature of a ferroelectric film.
A latch circuit is comprised of two inverters that are cross-coupled. The connections between the two inverters are provided by metal wiring resources such that a gate node of one inverter is coupled to a drain node of the other inverter. Straight above the two inverters, cross-connects are formed between metal wiring and contacts such as gate nodes and drain nodes. This structure makes it difficult to form a ferroelectric capacitor between the layer in which an inverter circuit is formed and the layer in which metal wiring is formed.
Conventional nonvolatile SRAMs cope with structural limitations as described above by growing the ferroelectric film at low temperature (450 degrees Celsius) by the MOCVD (metal-organic CVD) method after the process of forming metal wiring. In general, the quality of a film grown at low temperature is not as high as that of a film grown at high temperature (about 650 degrees Celsius). This causes an increase in the leak current between electrodes and the degradation of polarization characteristics of ferroelectric hysteresis, giving rise to new problems that need to be attended to.
Accordingly, there is a need for a stacked-layer structure of a nonvolatile semiconductor memory device for which a stable ferroelectric film can be formed while taking into account the cross-coupling connection of a latch circuit. There is also a need for a method of forming such a stacked-layer structure.
Moreover, there is a need for a stacked-layer structure of a nonvolatile semiconductor memory device for which a stable ferroelectric film can be formed while taking into account the cross-coupling connection of a latch circuit, and by which the size of the nonvolatile SRAM cell is reduced. There is also a need for a method of forming such a stacked-layer structure.